Conventionally, power conversion devices such as inverters that can convert DC power to AC power with an arbitrary frequency have been widely used for drive control of electric motors. An inverter can turn ON and OFF a semiconductor switching element on the basis of a voltage command value to output power with a desired frequency, thereby to control an electric motor drive current. PWM control to determine switching timing of the element with a pulse width modulation (PWM) signal is often used for the control of the switching element of the inverter.
This PWM control includes asynchronous PWM control to determine the switching timing by simply comparing a signal of the voltage command value and a signal of a carrier wave, and create a PWM pulse.
Further, as technologies other than the above-described basic asynchronous PWM control, there are PTL 1 and PTL 2.
PTL 1 discloses a technology of synchronous PWM control, in which “a synchronous PWM determination device 46 determines the number of synchronization in synchronous PWM control, and a synchronous PWM phase controller 48 determines a frequency of a triangular wave carrier according to the number of synchronization and determines timing of switching the number of synchronization. A PWM control unit 40 performs synchronous PWM control for a phase of a voltage command to synchronize a phase of the triangular wave carrier (see “Solution” in Abstract)”.
Further, PTL 2 discloses a technology of PWM control, in which “an inverter control circuit determines a phase of a sine wave reference signal by a phase determination unit 28, performs feedback control only in a period in which a converted AC current waveform is positive and outputs a target current, stores the waveform output in the previous positive period to a storage unit 25 and inverts and outputs the waveform in a negative period, and performs control to make the magnitude of currents to be output in the positive period and in the negative period equal (see “Solution” in Abstract)”.